Long-term Mid-infrared Color Variations of Narrow-line Seyfert 1 Galaxies

Abstract We present a systematic investigation of long-term mid-infrared (MIR) color variability in 1718 narrow-line Seyfert 1 galaxies (NLSy1s) using 14 yr of Wide-field Infrared Survey Explorer/NEOWISE monitoring data. Through Pearson correlation analysis between photometric magnitude and color, we identify (1) a radio-quiet NLSy1 (RQ-NLSy1) population comprising 230 bluer-when-brighter (BWB) sources, 131 redder-when-brighter (RWB) sources, and 1323 objects showing weak or statistically insignificant color variations; and (2) a radio-loud NLSy1 (RL-NLSy1) population containing 5 BWBs, 2 RWBs, and 27 sources with weak/no color variations. Our analysis reveals that the BWB tendency strengthens significantly in galaxies with redder mean MIR colors W1 -W2 and lower starlight contamination. Furthermore, this color-change pattern demonstrates that the most bolometric luminous sources exhibit the most pronounced BWB behavior. While similar trends exist for black hole mass and Eddington ratio, bolometric luminosity appears to be the primary physical driver. Potential origins of these variations (e.g., host galaxy contribution, accretion disk variability, and dust reprocessing) are discussed. We conclude that temperature-dependent dust reprocessing dominates the observed BWB, RWB, and no/weak variation patterns. This interpretation may also apply to similar MIR color variations (MCVs) observed in other extragalactic MIR transients, such as tidal disruption events, ambiguous nuclear transients, and changing-look AGNs. In addition, we find no significant difference in long-term MCVs between RL-NLSy1s and RQ-NLSy1s; however, RL-NLSy1s show significantly greater dispersion in intrinsic variability amplitude compared to RQ-NLSy1s due to jet-induced complexity, where nonthermal synchrotron emission from relativistic jets obscures thermal dust signatures.